Wolsong 1 retubed

The Wolsong 1 CANDU 6 nuclear power plant first came on line in 1983. Owned and operated by Korea Hydro and Nuclear Power (KHNP), the reactor has achieved a lifetime capacity factor of 86.2%, making it one of the top-performing reactors in Korea. Wolsong 1 is the first of a four-unit CANDU plant, the largest CANDU facility outside of Canada. In June 2006, KHNP signed a contract with Atomic Energy of Canada Ltd. (AECL) to initiate a retube project for the Wolsong 1 reactor. The commercial operations of AECL have recently been purchased by a wholly- owned subsidiary of SNC-Lavalin Inc, which operates as Candu Energy Inc.

CANDU reactors are designed to operate economically and reliably for up to 60 years, with an extended outage at mid-life to replace fuel channels. This outage also allows operators to refurbish other key plant components to ensure safe operation of a high-performing reactor for up to another 30 years.

Retubing the reactor involves removing and replacing feeders, fuel channels and calandria tubes (CTs); a CANDU 6 reactor has 760 feeders, 380 fuel channels and 380 calandria tubes (see diagram, below). Each calandria tube is approximately six metres long by 13 centimetres in diameter. Made of zirconium alloy, it houses the reactor’s fuel channels. The fuel channels connect to end fittings on each fuel channel assembly to circulate heavy water coolant between the reactor and steam generators. Fuel channels are attached via end fittings to feeder pipes that transport heavy water coolant to the reactor from the steam generators. Remotely-controlled tools and complex multi-task machines are required to conduct the retube work safely, due to the highly radioactive environment.

Cutaway of a Candu 6 calandria

Developing the plan

Work on the Wolsong 1 retube project began well in advance of the plant shutdown in the spring of 2009. Conceptual work on tool design, process development and permanent plant component procurement began immediately after the contract was signed in the summer of 2006.

Each of the ten steps of a retube (see box, below) is made up of several individual work series that carry with them a schedule and a process that must be followed in exacting detail. Every step was treated as a mini-project with a dedicated series lead responsible for preparations to ensure work went seamlessly on the reactor face.

Project management up front was key, considering:?that the project was more than 10,000 km away from critical support at the designers’ home office; the need to move, house and support more than 200 people and their families; the 5500 items in the Wolsong 1 retube toolset; and 1500 crates of permanent plant components.

Many of the Wolsong 1 tool designs were used at previous retube projects such as Point Lepreau, and rebuilt, or retrofitted, for Wolsong, based on the lessons learned along the way. For instance, the volume reduction system, a major piece of equipment, was modified between projects to increase functionality and performance.

Not only were tooling changes made, but tooling process changes were made as well, said John Drossis, director, retube engineering. “We learned that it is very effective for the experts on a particular piece of tooling to follow it right from development and testing to a project, and then home to receive the tool, refurbish it based on lessons learned, and finally travel on to the next project.” The tool leaders carry with them intimate knowledge of the intricacies and idiosyncrasies of each piece of tooling and pass on that knowledge to the training team on the ground.

Getting started

A small advance team arrived in Korea in the summer of 2008, with the majority of the team following in early 2009. The first order of business was to build mock-ups of the reactor and feeders to allow the testing of tools and processes. KHNP began the Wolsong 1 outage in April of 2009 and the reactor vault was turned over to the project team on 15 June, 13 days ahead of schedule; the team had already begun vacuum drying the primary heat transport system.

“This got the project off to a good start for all parties,” recalled Andrew Johnson, general manager responsible for the Wolsong retube project. “Our drying system was efficient in recovering heavy water and ensured low tritium levels so work in the vault could be conducted without needing plastic suits for personal protection.”

The removal of original components began in earnest in July, with the removal of the closure plugs and feeders. Once feeders had been removed, heavy-duty moveable platforms and work tables were installed on either side of the reactor face. This allowed tools to be aligned to the reactor face with high precision, so they could enter and cut pressure tubes.

The large-volume reduction system removed pressure tubes from the reactor core one at a time, and then crushed and cut them into 10 cm-square coupons directly on the reactor face, before releasing them into storage containers. The storage containers were transported via shielded flasks to purpose-built waste storage canisters.

The volume reduction system installed in the mockup building

The volume reduction system, set up for testing

Calandria tubes are normally held in place in a CANDU reactor by rolled joints at each end. To remove them, shock heating was used, quickly heating up the calandria tube insert and freeing it from the joint. The volume reduction system was reconfigured to cut and crush these components for disposal.

This phase of the project was complex due to the radioactive work environment. The multifaceted tooling systems were operated by workers receiving remote instruction from the retube operations centre (ROC). Making use of sophisticated camera equipment, ROC technicians were able to guide workers to complete tasks quickly and safely, reducing overall radiation exposure.

Once the original components had been removed from the reactor, the project team turned its attention to inspecting and cleaning the calandria, ensuring that no foreign material was left inside the reactor. Bellows were also inspected during this phase. Bellows connect fuel channels to the reactor structure, thus enclosing the annulus gas system while also accommodating fuel channel elongation.

Exact measurements were made of the reactor tube sheets—both profile and distance between the tube sheets—using laser measuring tools. The tube sheet bores were polished and inspected to ensure they were suitable to install new calandria tubes.

Installing new components

Projects of this magnitude are not without their technical challenges, and despite the best preparations, the project encountered tooling performance issues, which led to significant remedial work activities.

“There were significant learnings in this period,” said Bill Pilkington, senior vice president, projects and services. “Every challenge brings with it an opportunity to learn and improve. The changes we implemented as a result of our experience have proven successful in ensuring CANDU reactors can be successfully retubed. This is knowledge we will take forward in future projects.”

Rolled joints were made for each calandria tube and then leak-tested to the highest standards, confirming joint integrity. Once successful joints were made on each end of all 380 calandria tubes, fuel channel assembly installation began. Before a fuel channel assembly can be installed, an end fitting subassembly is first made in a clean room. The Wolsong retube project established a dedicated clean room to prepare calandria tubes, pressure tubes and end fitting subassemblies. The clean room was responsible for just-in-time delivery of subassemblies to the reactor face.

One of the most demanding steps in the entire retube process, fuel channel assembly installation, requires many work steps. It was here that KHNP and sub-contractor Korea Plant Service & Engineering (KPS) really proved their ability to work as an efficient and integrated team.

With the higher precision and quality assurance required for this portion of the work, a less-automated, but highly-skilled manual approach was necessary. “There are many check points in installation,” explained Jamie Hopkins, project director. “Like a new build, each movement is confirmed to ensure components are installed correctly and to record as-built measurements.”

The pressure tube/end fitting subassemblies were inserted into the calandria tubes working from one side of the reactor, and then the annulus spacers were installed and carefully positioned. The position of the spacers was verified by eddy current testing. Next, the second end fitting was aligned and installed on the other end of the pressure tube. Alignment and positioning of the pressure tube and end fittings were carefully controlled and checked before  the fuel channel was fixed in position by welding the bellows and installing positioning hardware. There were literally hundreds of separate steps that had to be conducted to install each fuel channel; the coordination of this work was an area in which KPS demonstrated its first-class skills.

Project organization

Within the project, teams were developed for each series. Each team developed and tested processes so everything was ready and working when needed for on-face reactor work. A specialized tool testing and maintenance unit was established to make sure that tools worked properly even before testing and training on the mockup to ensure best use of time. That same team was available around the clock to repair tools on the reactor face, reducing downtime further.

The project developed a ‘war room’ early on to ensure quality management decisions could be made quickly and with all needed input. This required all managers to work side-by-side in a single room. By ensuring all key management controls were working together, and with the ability to view activities in, and communicate with, the ROC, the war room helped the project maintain its focus on quality, productivity and above all, safety.

Voice and visual recording systems provided ROC technicians with at least a dozen views of each reactor face. In fact, in most cases, ROC technicians had a far better view of the on-face work than the operators actually conducting the work, due to both camera angles and the sheer size of the precision-controlled tooling systems being operated. “The speed by which direction could be determined and passed on to the workers was one of the keys to our success,” said Hopkins. “It enabled us to increase productivity and reduce radiation exposure at the same time.”

Additional camera angles also enabled ROC technicians to ensure that tools had operated as intended. For instance, during shield plug removal, the tooling system was designed to pull out the shield plug and then release it into a flask. ROC technicians were able to determine in one instance that the tool had seized up and had not completed the ‘drop’. They were then able to immediately engage the maintenance unit to repair the tool.

ROC technicians also had access to video communications with the support team in Mississauga. Although the project was fully staffed in Korea, expert advice was still required from the Mississauga (Canada) head office. Daily telephone calls and video conferences were held among the project staff, engineering staff and tooling staff, in spite of the 12-hour time difference. That time difference often proved advantageous, with the Korean team setting out their requirements at the end of their day. By the time they arrived back at work the next morning, they found a solution.

A successful conclusion

The Wolsong reactor retube outage was completed in 839 days, during which KHNP also completed a wide range of other plant refurbishment activities, including DCC?(plant control computer) replacement, probabilistic safety review follow-up actions, safety system upgrades and aging component replacement.

AECL and KHNP staff celebrate the Wolsong 1 return to full power

The vault at Wolsong Unit 1 was transferred back to KHNP on 8 March 2011. They then took the lead on return-to-service operations, and the unit was successfully returned to full power on the Korean electrical grid on 18 July, marking the first CANDU 6 reactor to be retubed and restarted.

With the sale of AECL’s commercial operations to SNC-Lavalin in October 2011, a new company was formed. Candu Energy Inc. has retained the vast majority of key staff involved in the Wolsong life extension project. Their experience and expertise is being brought to bear in planning for future projects. The Candu team is now working on preparations for a life extension project for Argentina’s Nucleoeléctrica Argentina Sociedad Anónima (NA-SA). The retube outage at NA-SA’s Embalse plant is currently scheduled to begin in late 2013.

This article was first published in the January 2012 issue of Nuclear Engineering International